I. Field of the Invention
The present invention relates to an apparatus for controlling steering of wheels of a vehicle and, more particularly, to an apparatus for controlling steering of wheels of a vehicle in which a high correction capability of variables representing vehicle behavior is provided against disturbances such as side and gusty winds, and high response for changes in vehicle behavior in response to a steering input such as rapid steering or vehicle turning with a large radius.
II. Description of the Prior Art
Traveling performance and steering stability of a vehicle primarily depend on a yaw rate (i.e., angular velocity generated around the center of gravity when the vehicle is viewed from the top) of the vehicle responding to a steering input, i.e., a steering wheel operation, or a lateral acceleration (i.e., an acceleration acting on the center of gravity of the vehicle in a direction perpendicular to the vehicle traveling direction, i.e., an acceleration parallel to the traveling direction of the vehicle).
It is ideal to guarantee safety for causing the vehicle to travel straight according to the will of a driver without being influenced by disturbances such as a side wind or a road surface friction coefficient when the vehicle is to quickly respond to the steering input or is to be driven straight.
In order to provide both dynamic characteristics and stability described above, Japanese Laid-Open Patent Publication 60-161266 describes an apparatus obtained by combining feed forward control and feedback control. In this apparatus, the following two methods are used. The first method is to control traveling performance and stability by a value obtained such that a differential value of a difference obtained by subtracting a coefficient multiple of a yaw rate or a lateral acceleration from a steering wheel angle is added to the difference, as shown in FIG. 2. The second method is practiced as follows, as shown in FIG. 3. A value is obtained by multiplying a coefficient with a differential value of a steering wheel angle and is added to the steering angle to obtain a sum. The sum is used as a feed forward signal for auxiliary steering of the rear wheels, and the vehicle behavior is used as a negative feedback signal for auxiliary steering of the front wheels. The drawback of feedback control is canceled with that of feedback control.
In the auxiliary steering apparatus described above, when a driver rapidly turns the steering wheel in order to avoid an obstacle, auxiliary variables representing vehicle behavior are rapidly increased by a control value influenced by the differential value, and the vehicle is rapidly turned. Heavy mental load imposed on the driver is caused by rapid vehicle turning. In the worst case, the driver excessively turns the steering wheel, the vehicle loses stability, and a traffic accident may occur. When the vehicle travels along a curved road having a relatively large radius (e.g., a highway), the differential value is rarely influenced by the vehicle behavior. The feed forward signal is decreased, and understeering in the vehicle is caused as a whole. As a result, traveling performance of the vehicle is degraded. When an external force acts on the vehicle due to a side wind, ruts, or steps, the driver tends to turn the steering wheel with a guess so as to respond to accidental disturbances. Such steering increases the differential value, and oversteering may be caused to leading to unexpected vehicle behavior.
The above disadvantage of the conventional auxiliary steering apparatus is caused by feed forward control using a steering wheel angle and a value obtained by multiplying a differential value of the steering wheel angle with a coefficient. The influence of the differential value is increased as compared with the normal steering apparatus, and desired control cannot be performed. When the coefficient multiplied with the differential value is decreased, a total gain is reduced and the control system in the vehicle is substantially the same as that of feedback control. Optimal control cannot be performed by the above-mentioned auxiliary steering apparatus.
In the auxiliary steering apparatus shown in FIG. 3, independent control operations are performed for the front and rear wheels. Behavior of the front and rear wheels is independently performed and the required control apparatus is complicated, thus increasing the overall weight and the like of the vehicle and hence failing to obtain the desired control effect.